RAID (redundant array of independent (or inexpensive) disks) provides large disk capacity with very high reliability. This technology is now also referred to generally as “disk array” technology. RAID employs multiple physically separate disks, all accessed through a single array controller which makes the separate drives appear as a single composite drive. Typically the separate disks will be connected through a synchronization cable, so that the spinning platters of all drives are locked into the same rotational phase relative to each other. In different configurations, this approach can be used to achieve higher reliability, faster maximum data rate, and/or shorter maximum access time.
To achieve higher reliability, information is stored so that each disk holds a check or parity byte for the corresponding bytes in the other disks. Thus, if one disk fails, its information can be reconstructed from the information and parity on the other disks. For example, information can be reconstructed to a hot spare or hot standby disk which is used as a failover mechanism to provide reliability in system configurations.
The hot spare is active and connected as part of a working system, and when one disk fails, the hot spare is switched into operation. For example, both hardware and software RAIDs with redundancy may support the use of hot spare drives, e.g., a drive physically installed in the array which is inactive until an active drive fails. In implementation, the RAID system can automatically replace the failed drive with the spare. This reduces the mean time to recovery (MTTR), though it does not eliminate it completely. In fact, subsequent additional failure(s) in the same RAID redundancy group before the array is fully rebuilt can result in loss of the data; rebuilding can take several hours, especially on busy systems.